JPH0723510B2 - Method for producing hot coil of boron-containing austenitic stainless steel - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for producing a hot coil of boron-containing austenitic stainless steel.

[Conventional technology]

Boron-containing austenitic stainless steel is suitable as a thermal neutron shielding material because it has a characteristic of having a large thermal neutron absorption cross section and is excellent in corrosion resistance. However,
Since the precipitation of intermetallic compounds, mainly boride, which is a combination of B and Fe or Cr in steel, significantly impairs hot workability,
Steel strips are difficult to manufacture.

Japanese Patent Publication No. 57-45464 teaches that the hot workability of B-containing stainless steel is improved by the addition of Al.

JP 55-89459 teaches that the addition of V to B-containing stainless steels improves the hot workability by improving the morphology of the borides.

However, it is difficult to produce a boron-containing austenitic stainless steel strip by an ordinary hot rolling mill, regardless of the teaching of the effect of improving the hot workability by adding these alloying elements. In fact, none of the above publications describe a method for producing a B-containing stainless steel strip.

In the Japanese Patent Application No. 61-240244, the present inventors set B to 0.2 to
An austenitic stainless steel slab or ingot containing 1.2% by weight is heated to a temperature of 1150 ° C to 1200 ° C, and the rolling end temperature is set to 1000 using a rough rolling mill and a finish rolling mill.
The invention of a method for producing a hot-rolled steel strip of a boron-containing austenitic stainless steel, which comprises hot rolling at a temperature of ℃ or higher, was proposed.

[Object of the Invention]

The present invention is intended to further improve the method proposed in Japanese Patent Application No. 61-240244. That is, the present inventors have further researched the hot workability of boron-containing austenitic stainless steel, but in the previously proposed method, crack defects may occur in the hot coil, and the cause thereof has been investigated. Along with this, the purpose is to prevent this.

[Structure of Invention]

The present invention provides a method for producing a hot coil of a boron-containing austenitic stainless steel without cracks and flaws. The gist of the present invention is to provide an austenitic stainless steel ingot containing 0.2 to 1.2% by weight of B. 20 to 70% by manufacturing and heating this steel ingot to a temperature of 1150 ℃ to 1200 ℃
20 ~ 70% by heating the obtained slab of slab to a temperature of 1150 ℃ to 1200 ℃.
It is characterized in that, after undergoing an intermediate slab manufacturing process in which reduction is performed, the intermediate slab is heated to a temperature of 1150 ° C to 1200 ° C and hot-rolled to a steel strip. In the case of continuous casting, a continuous casting slab of austenitic stainless steel containing 0.2 to 1.2% by weight of B is manufactured, and the continuous casting slab is heated to a temperature of 1150 ° C to 1200 ° C for 20 to 70 ° C. %
The method is characterized in that, after undergoing an intermediate slab manufacturing process in which reduction is performed, the intermediate slab is heated to a temperature of 1150 ° C to 1200 ° C and hot-rolled to a steel strip.

[Detailed Description of the Invention]

The present inventors conducted various test studies on the hot workability of the B-added austenitic stainless steel.
The hot workability of the added steel decreases significantly with the increase of strain rate, but once forged at high temperature (that is, reduction)
It was found that the rupture due to high-temperature tension is remarkably improved if the heat treatment is appropriately performed after the heat treatment. More specifically,
It has been clarified that the fracture of the B-added steel due to high temperature tension occurs at the starting point of the boride or at the boundary between the boride and the matrix. However, when the forging (reduction) and heat treatment are properly performed, It was found that the hot workability was remarkably improved as a result of the fact that the compound can be finely dispersed and the location that is the starting point of fracture is finely dispersed. The present invention realizes this finding in an actual hot coil production line. Instead of directly hot rolling a slab of slabs or a continuous cast slab as usual, a hot coil is produced in the middle. The process of manufacturing the intermediate slab is added, and after the forging effect and heat treatment effect are fully exerted, the final hot rolling is performed. According to this method, the boron-containing austenitic stainless steel containing a large amount of B is used. However, it was found that a hot coil with no cracks can be obtained stably.

The present invention is directed to an austenitic stainless steel having a B content of 0.2 to 1.2% by weight, which requires at least 0.2% by weight of B to obtain the thermal neutron damping effect by the addition of B. The workability and toughness of steels with a B content exceeding 1.2% by weight are extremely poor, and the thermal neutron decay effect is often sufficiently satisfied without the need to add such a large amount of B. Is. In addition to B, it goes without saying that the elements contained in the steel required to form an austenitic stainless steel are contained in the steel, and virtually all types of base austenitic stainless steel are covered by the method of the present invention. It can be.

Fig. 1 shows the test temperature and the cross-sectional shrinkage ratio of various slabs for the test steel Nos. 2 and 3 shown in Table 1 below when they were subjected to a high temperature tensile test at a strain rate of 1 / sec. It shows the relationship with. Further, FIG. 2 shows the same relationship as in FIG. 1 except that the strain rate is 50 / sec. Here, the strain rate of 1 / sec (Fig. 1) corresponds to the normal strain rate of hot working in rough rolling during the production of lump slabs and intermediate slabs, and strain rate: 50 / sec (second The figure) corresponds to the normal strain rate in finish hot rolling.

The slabs tested are as follows.

● mark: continuous cast slab of test steel No. 2, ■ mark: slab obtained by rolling down the steel ingot of test steel No. 3 at a reduction rate of 67%, ▲ mark: test steel No. 2 Intermediate slab obtained by heating the continuous cast slab of No. 3 at 1180 ° C and reducing it at a reduction rate of 30%, ◆: Slab slab obtained by reducing the steel ingot of sample steel No. 3 at a reduction rate of 65% Is further heated to 1180 ° C and rolled at a reduction rate of 22% to obtain an intermediate slab.

As is clear from the results shown in Fig. 1, even in the case of hot working with a low strain rate such as rough rolling, the cross-sectional shrinkage rate is relatively high even with boron-containing austenitic stainless steel, and it can be said that it is a continuous cast slab. In particular, in the range of 1100 to 1200 ° C, the deformability is good and an intermediate slab with no cracks can be obtained. The same applies to the case of obtaining a slab or an intermediate slab from a steel ingot. The deformability of the intermediate slab is higher than that of the original slab or continuous cast slab.

However, as shown in Fig. 2, the cross-sectional shrinkage rate becomes low under the high strain rate equivalent to that of finish hot rolling. This tendency is particularly strong with continuous cast slabs and with agglomerated slabs. Therefore, the deformability of the continuous cast slab as it is or the as-cast slab manufactured from steel ingot is inferior, and if it is hot-rolled for finishing, cracks are likely to occur in the hot coil. This tendency is particularly strong in finish hot rolling at 1000 ° C or lower. In contrast, the intermediate slab manufactured from the continuous cast slab and the intermediate slab manufactured from the steel ingot through the slab slab have improved deformability even when the strain rate is high.
This tendency is particularly remarkable in the range of 1100 to 1200 ° C.

As shown in both figures, the deformability of boron-containing austenitic stainless steel at high temperatures decreases with decreasing temperature, so it is desirable that the heating temperature during hot working be as high as possible. However, as shown in the results of both figures, it is clear that the deformability of boron-containing austenitic stainless steel extremely decreases above 1200 ° C and almost no processing is possible. That is, the processing temperature is 12% both when the strain rate is slow as in rough rolling and when it is fast as in hot rolling for finishing.
It should not exceed 00 ° C. This is 1200 boride
It is thought that this is due to melting at a temperature above ° C.

In the above-mentioned Japanese Patent Application No. 61-240244, a slab or ingot of B-containing austenitic stainless steel is heated to a temperature of 1150 ° C or higher and 1200 ° C or lower, and the rolling end temperature is set to 1000 by using a rough rolling mill and a finish rolling mill. It was proposed to carry out hot rolling so that the temperature becomes ℃ or higher. This means that the continuous casting slab and the agglomerated slab marked with ● or ■ are heated to 1150 ° C to 1200 ° C and hot rolled at a hot rolling end temperature of 1000 ° C. In this case, hot rolling is possible, but cracks (particularly ear cracks) may occur in the hot rolled steel strip obtained. However, as shown by the ▲ and ◆ marks in Fig. 2, the intermediate slab was manufactured (as shown in the results in Fig. 1).
This can be done at a temperature of 1150 ℃ to 1200 ℃).
If it is heated to 50 ℃ -1200 ℃ and hot-rolled at 1000 ℃, the hot coil can be manufactured without cracks because it has better workability than those marked with ● or ■. 1 and 2 show sufficient deformability at a temperature of 1100 ° C or higher, but at 1150 ° C, there is a tendency to show better deformability. Therefore, in carrying out the method of the present invention, a lump slab having a low strain rate, its intermediate slab,
Furthermore, when manufacturing an intermediate slab from a continuous cast slab, the heating temperature is preferably in the range of 1150 to 1200 ° C, and the heating temperature for finish hot rolling from these is preferably in the range of 1150 to 1200 ° C. As for the finishing hot rolling temperature, as shown in the results of Fig. 2, when the temperature is lower than 1000 ° C, the deterioration of the deformability becomes large, so it is preferable to finish the hot rolling at a temperature of 1000 ° C or higher, For this reason, the finishing hot rolling temperature is 1000 ° C.
The above is preferable.

As described above, if the process for producing the intermediate slab is adopted, the deformability of the boron-containing austenitic stainless steel in the finish hot rolling can be enhanced. This can be considered as follows. In other words, the decrease in deformability that appears in boron-containing austenitic stainless steel is due to the presence of boride, but in particular, the extreme segregation of boride during solidification further promotes the decrease in deformability. When the process for manufacturing the intermediate slab is added, the forging effect due to the re-pressing and the heat treatment effect due to the reheating appear, the segregation of boride is relaxed, and a structure in which boride is finely dispersed is formed, and this microstructure is maintained. Then, the deformability is large, and if hot rolling is performed in this state, hot coils can be manufactured without cracks.

Before the finish hot rolling to the steel strip, the more the number of times the intermediate slab is manufactured and processed, the more the boride segregation is relaxed and the deformability is improved. However, it is not preferable to unnecessarily increase the number of times of machining, because it increases the manufacturing cost. Therefore, it is appropriate to use slab production and intermediate slab production twice in the steel ingot method, and intermediate slab production in the continuous casting method. The object of the present invention can be achieved only once, and only once. The total rolling reduction per production of the slab or intermediate slab must be 20 to 70%. This is because when the rolling reduction is less than 20%, the effect of dispersing boride and relaxing segregation is not obtained, while when the rolling reduction exceeds 70%, the cumulative strain increases and the temperature decreases with the increase of the processing time. This reduces the deformability and may cause cracking.

〔Example〕

Continuously cast slabs and ingots of boron-containing austenitic stainless steel having the chemical composition shown in Table 1 were produced.

These slabs or steel ingots were manufactured into intermediate slabs according to the slab production conditions and intermediate slab production conditions shown in Table 2, and then hot rolled into steel strips under the hot rolling conditions shown in Table 2. For comparison, Table 2 also shows an example of hot rolling into a steel strip without producing an intermediate slab. The state of cracks and flaws in the obtained hot coil was investigated, and the results are shown in Table 2.

As can be seen from the results in Table 2, in steel ingots, hot rolling after slab production + intermediate slab production has no cracks and cracks compared with post-hot rolling only slab production. Good results have been obtained. In addition, in the case of continuous cast slabs, better results are obtained when hot rolling is performed after the intermediate slab is manufactured than when it is directly hot rolled without manufacturing the intermediate slab. Therefore, according to the method of the present invention, it can be seen that a hot coil of boron-containing austenitic stainless steel without cracks can be satisfactorily manufactured.

[Brief description of drawings]

FIG. 1 is a diagram showing the relationship between the test temperature and the cross-sectional shrinkage rate when a boron-containing austenitic stainless steel was subjected to a high temperature tensile test at a strain rate of 1 / sec. FIG. 2 is a diagram showing the relationship between the test temperature and the cross-sectional shrinkage rate when a boron-containing austenitic stainless steel was subjected to a high temperature tensile test at a strain rate of 50 / sec.

Claims (2)

[Claims] 1. A steel ingot of austenitic stainless steel containing 0.2 to 1.2% by weight of B is produced, and the ingot is heated to a temperature of 1150 ° C. to 1200 ° C. to reduce it by 20 to 70%. The lump slab manufacturing process and the intermediate slab manufacturing process in which the obtained slab mass slab is heated to a temperature of 1150 ° C to 1200 ° C and subjected to 20 to 70% reduction, and the intermediate slab is heated to 1150 ° C
A method for producing a hot coil of boron-containing austenitic stainless steel, which comprises heating to a temperature of 1200 ° C or lower and hot rolling to a steel strip. 2. A continuous casting slab of austenitic stainless steel containing 0.2 to 1.2% by weight of B is manufactured, and the continuous casting slab is heated to a temperature of 1150 ° C. to 1200 ° C. to reduce it by 20 to 70%. A method for manufacturing a hot coil of boron-containing austenitic stainless steel, which comprises heating an intermediate slab to a temperature of 1150 ° C. or higher and 1200 ° C. or lower and then hot rolling the steel strip after an intermediate slab manufacturing process.